This disclosure is related to a heat treatment method and to components heat treated according to the method.
Superalloys are metallic alloys for elevated temperature service, generally based on group VIIA elements of the periodic table, and are used for elevated temperature applications where resistance to deformation and stability are desired. The common superalloys are based on nickel, cobalt or iron. Nickel-iron base superalloys such as, for example Alloy 706 are generally employed as materials of construction in gas turbine engine components such as rotor discs (hereinafter rotors) and spacers.
As a result of the demand for improved performance and efficiency, the components of modern gas turbine engines operate near the limit of their properties with respect to temperature, stress, and oxidation/corrosion. Due to these aggressive operating environments, the superalloys from which the components are made generally possess a combination of exceptional properties including high strength capabilities at elevated temperatures greater than or equal to about 700° F. In particular, nickel-iron base superalloy articles suitable for components such as turbine rotors and discs must possess superior low cycle fatigue strength because of repeated cycling between full engine power and idle. This repeated cycling induces thermomechanical stresses within the engine. It is generally desirable for such superalloy articles to possess superior low cycle fatigue strength in order to withstand such conditions. In current gas turbine rotor designs, life of the rotor can be limited by the low cycle fatigue capability of the material.
There are two known heat treatment processes that are prescribed by International Nickel Company (INCO), the inventor of the Alloy 706. The two known heat treatment processes are heat treatment A and heat treatment B respectively. Heat treatment A is recommended for optimum creep and high temperature rupture properties, while heat treatment B is recommended for applications requiring high tensile strength.
Heat treatment A comprises a solution treatment at 1700 to 1850° F. for a time commensurate with the section size, followed by a first air cooling. The first air cooling is followed by a stabilization treatment at 1550° F. for three hours, followed by a second air cooling. Following the second air cooling is a precipitation treatment at 1325° F. for 8 hours. The object is then cooled in a furnace at an average rate of 100° F./hr to 1150° F. where it is held for 8 hours. The cooling in the furnace is followed by a third air cooling.
Heat treatment B comprises a solution treatment at 1700 to 1850° F. for a time commensurate with the section size followed by a first air cooling. The first air cooling is followed by a precipitation treatment at 1350° F. for 8 hours followed by cooling in a furnace at an average rate of 100° F./hr to a temperature of 1150° F. where it is held for one hour. This is followed by a second air cooling.
While heat treatment A is recommended for optimum creep and high temperature rupture properties and heat treatment B is recommended for applications requiring a high tensile strength there are no treatments that improve the low cycle fatigue of components manufactured from superalloys. It is therefore desirable to provide a heat treatment for turbine rotors manufactured from superalloys that facilitate an improvement in the low cycle fatigue capability of the rotor.